Lead is widely present in the environment, due to its natural occurrence, as well as human activities. It is considered as one of the most common pollutants. The main sources of lead contamination in the environment come from:
- The combustion of fossil fuels
- Lead insecticides, which are now prohibited, however, the presence of lead can still be found in some former agricultural areas
- Inorganic fertilisers, which may contain lead as an impurity
- Emissions from smelters
- Sewage sludge
- Lead-based paint pigments
- Storage batteries
Plants are capable of absorbing and translocating lead in their tissue at different concentrations. This property is used in mineral prospecting for discovering ore deposits, and more recently for phytoremediation. Lead is absorbed by plants through root systems, or can be deposited on their leaves and stems as a result of air contamination. Lead, in the form of Pb2+ and at high concentration, is toxic to most living beings. This form of lead is not biodegradable, and once the soil is contaminated, it remains as a long-term source of lead exposure, and consequently may be absorbed by plant tissue. Lead accumulates in the top 20 cm of soil, and is highly immobile, which complicates its remediation processes. Lead is tested in plant tissue samples for various industries such as agriculture, food, mining, and phytomedicine. Plant tissue analysis for lead is a routine procedure in most laboratories. Generally, the sample is dried at 65-85 degrees C, then used dried or pre-ashed at 450-500 degrees C. The material is further digested in nitric acid, and the solution is then analysed using ICP-MS (Inductively Coupled Plasma Mass Spectrometer), ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer) or AAS (Atomic absorption spectroscopy). A new alternative analysis involves direct solid sampling, with the plant tissue slurry being introduced directly into instruments, such as a Graphite Furnace Spectrometer (ContrAA by Analytik Jena). This procedure eliminates errors caused by manual operations, and considerably reduces cost and time of analysis. Numerous studies have estimated the concentration of lead in various fruit and vegetables. Depending on the country of origin, numbers can vary. For example, the concentration of lead in fruit collected from Libyan markets was: banana (0.1 ppm), peach (0.25 ppm), strawberries (0.53 ppm), apple (0.2 ppm) and grapes (0.4 ppm). However, lead intake from these fruits, based on an average body weight of 60 kg, was still within safety limits set by the World Health Organisation - 0.2 ppm.
When an appropriate analytical technique has been chosen, it is then also important to choose correct reference materials, in order to validate analytical results and cover the full concentration range of samples.